fix(experimental): Add experimental LDScopedClient for incrementally building and propagating contexts (#297)

You can now create a "scoped client", a wrapper of the client that will
use the specified context for all operations, including feature flag
evaluations and event tracking. This facilitates propagating a context
down to all logic that is related to a scoped piece of logic, like a web
request.

An `LDScopedClient` implements most of the same methods as `LDClient`,
but without the `context` argument.

A scoped client is mutable, to facilitate incrementally building up a
multi-context representing the current scope. You can add new contexts
(as long as they aren't replacing an existing kind) with
`LDScopedClient.AddContext`, and all contexts added so far will be
combined into a multi-context whenever the scoped client is used.
`LDScopedClient.OverwriteContextByKind` is offered as an escape hatch if
you need to update existing data.

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Co-authored-by: Matthew M. Keeler <[email protected]>

Author: aengelberg